Earth History Lecture 20

The Miocene Epoch-The Turning Point

1.  The Miocene Epoch  in the Cenozoic Era ranges in time from about 23.8 million years ago to 5.3 million years ago.

The Miocene begins warm (although not as warm as the Eocene was!) and ends cool.  We can think of it as the turning point for Earth's climate, because in Mid-Miocene, the global cool-down begins that ultimately leads to the glacial-interglacial cycle that dominates the Pleistocene Epoch.

2.  Features of the Miocene that we will focus on in this lecture are

a.  the eruption of the flood basalts that form the Columbia River Basalts, which causes a warm event in the mid Miocene because of additions of CO2 to the atmosphere

b.  the uplift of the Himalayas, Tibetan Plateau, and the Alps, which cause global cooling by weathering reactions that draw down CO2 levels in the atmosphere

c.  the consequence of the compressive collision of Africa with Europe-the drying out of the Mediterranean Sea in late Miocene time, last remnant of the Tethys Seaway

d.  the isolation of Antarctica after the Drake Passage opens, which initiates the permanent ice sheet formation on Antarctica

3.  The Miocene Epoch had a diverse fauna on land and sea, including:

a.   a top predator in the marine ecosystem-the giant shark Megalodon carcharadon, which reached lengths of 13 to 15 meters and preyed upon whales, among other sealife. 

b.  On land, diversification of the grazing mammals continued with the spread of grasslands.  The forested ecosystems gave way to grasslands with enclosed forests, and these grasslands supported a wide range of grazing and browsing mammals (niche partitioning again!)

4.  The Columbia River Basalts are the result of Flood Basalt eruptions of Mid-Miocene age

although not as large or long-lasting as the Deccan Traps or the Siberian Traps, they did release enough CO2 to drive a warm event in the mid Miocene

exposures of the Columbia River Basalts can be seen in Washington State, Oregon, and Idaho

5.  When we look at a map of the modern world, we can see there are some extensive orogenic belts-mountain building areas.  All of these were active in the Miocene, especially the late Miocene.  The Alpine-Himalayan Belt is an important one for this discussion, because of the very large geographic area that is the mountain chain and the Tibetan Plateau-uplifted dramatically in Miocene time.

6.  India's progressive tectonic motion through Cenozoic time ends in a collision with Eurasia about 10 million years ago, in the Miocene.  The ocean floor (sediments and crust) between India and Eurasia buckled up and folded into the uplifting crust.  There are no volcanos associated with this collision, because no subduction occurred.  The ocean floor sediments and crust are preserved as sedimentary and igneous rocks in the Himalayan Mountains (and the same is true for the Alps, as Africa collided with Western Europe).

a.  This results in marine sedimentary rocks with marine fossils located at great elevation in the Himalayas and the Alps (fossil seashells on Everest (Himalayas)  and the Matterhorn (the Alps)!)

b.  More importantly, this uplift of the mountains and the Tibetan Plateau drive large scale weathering reactions, and CO2 from the atmosphere is used in these reactions.  The gigantic size of the region uplifted means that alot of CO2 was used, with the result that atmospheric cooling occurred.

7.  The collision of Africa and the Arabian Peninsula with Western Europe also had a big effect besides uplifting the Alps-the openings to the Mediterranean Sea were pinched shut.  This resulted in the Mediterranean (last remnant of the old Tethys Seaway) drying out.  The Mediterranean is in an arid part of the world (lots of evaporation) and although rivers do supply water to the Mediterranean, most of the water comes from the Atlantic Ocean by way of the Gibraltar straits.  These straits closed in late Miocene, and the Mediterranean dried out.

a.  evidence for this is of two types-evaporite deposits in the sedimentary rocks below the modern Mediterranean sediments and in outcrop around the basin, and the downcut, much lower rivermouths (now filled with sediment) that supplied water to the basin during Miocene time. 

b.  The Mediterranean refilled when tectonic motion reopened the Gibraltar straits-the Atlantic refilled the basin.  This huge waterfall event is known as Gibraltar Falls, and is estimated to be at least 40x the size of Niagara Falls, and to have gone on for years in the refilling. 

c.  During the dried up phase, the landscape in the basin was marked by dunes and small brine pools, and was a harsh environment but still a potential landbridge for animals to cross from either continent.

8.  The opening of the Drake Passage, the strait between South America and Antarctica, occurred between 15 and 14 million years ago.  The effect was to finally isolate Antarctica from the other Gondwanaland continents that had all split up.  Antarctica, once isolated over the South Pole, was surrounded by the circumpolar current of the Southern Ocean.  This cold water was still warmer than the air temperature on Antarctica, and set up a "lake effect" (for those living near Lake Erie, this is immediately understandable) supply of snow and the permanent ice sheet began to grow. 

9.  Antarctica is covered by ice, but it is not one single sheet of ice.  There is a very large, thick (2-3 miles thick) layer of ice called the East Antarctic ice sheet, which is grounded on mountain ranges and continental rock, and there is a thinner, more variable West Antarctic Ice sheet, which is grounded only in part on rock, but also extends across ocean water and is held in check by the ice barriers such as the Ross Ice Shelf.  The West Antarctic Ice sheet has been identified as very sensitive to warming events, and the concern is that a collapse of this ice sheet might occur with increased warming, and if it does, that sea level will rise.  Sea level is a concern, because a large percentage of the human population lives within only 2 feet of sea level now, so an increase could be a big problem. 

10.  A way to understand how the West Antarctic Ice Sheet has responded in the past to natural conditions is to drill through the ice, down to the floor of the ocean, and into the rocks, going right back to Miocene time.  This project is underway with the Andrill Project.  Watch this YouTube video that introduces the project and the goals, and gives a view of the drilling itself.

                           Antarctica’s Ice on the Move-Antarctica’s Climate Secrets

 www.youtube.com/watch?NR=1&v=Ymxi5wsqtEU

11.  The discoveries so far (this project is still ongoing) include

the development of the West Antarctic ice sheet in less than a million years, between 15 and 14 million years ago

the identification of many collapses and rebuilding events of the West Antarctic Ice sheet, up to 60 of them, over the past 14 million years.  The most recent collapse and rebuild of the ice sheet was about 1 million years ago, in the middle of the Pleistocene Epoch.  Sea level rose during these collapses, which is important to know about. A small rise in sea level of 2 feet would not have been a big deal until now, with such a large population living in coastal areas within that range. 

12.  Watch this Youtube Video on the climate record from the Andrill Project.

Decoding Antarctica's Climate History - Antarctica's Climate Secrets

http://www.youtube.com/watch?v=oZHRwDHh4B0&feature=relmfu

13.  So the Miocene Epoch is really the turning point for Earth.  In the mid-Miocene (about 14-15 million years ago) following a brief warming event (Columbia River Basalts) the Earth starts the long descent into cooler conditions.  Important tectonic events that contributed to that were the Alpine-Himalayan Orogenic Belt activity (rise of the Alps, Himalayas, Tibetan Plateau) and the isolation of Antarctica, which initiated the formation of the ice sheets on that continent.

If you have time, there is a full length 50 minute video on the Andrill project that can be accessed with this link-enjoy!

http://www.pbs.org/wgbh/nova/earth/secrets-beneath-ice.html

Next Lecture-the Hominid Fossil Record which begins in late Miocene time.